1. Technical Field
The present disclosure relates to a method of mounting one conductive ball on each of a plurality of connection pads on a substrate.
2. Related Art
In the related art, in forming solder bumps on connection pads for flip-chip connection of a substrate such as a wafer or a circuit substrate, a solder paste is screen printed and reflowed. However, with an increase in density of a semiconductor device, the bump pitch needs to be reduced to a pitch as fine as 200 μm or further 150 μm. When a solder paste is screen printed at such a fine pitch, bridge between bumps or adhesion of the solder paste to a printing mask occurs. For this reason, formation of bumps at a fine pitch is difficult to perform with screen printing.
Thus, as a fine pitch bump formation method, there is a method in which solder balls are provided on connection pads and then reflowed.
As one method, there is a method to be performed on connection pads for board mounting on an external connection terminal side of Ball Grid Array (BGA), in which solder balls are chucked by a chucking jig and then are carried and mounted on the connections pads. On the connection pads, adhesive soldering flux is coated in advance. As a result, the solder balls adhere onto the connection pads, and fixed thereto. However, in this case, the solder ball diameter is as large as about 0.3 to 0.7 mm. In contrast, the solder ball diameter for fine pitch needs to be set at, at least 200 μm or less. In cases where the solder balls are thus fine, the solder balls agglomerate each other by the action of static electricity, and they cannot be accurately chucked on the chucking portion of the chucking jig under the influence of airflow. Further, the chucking portion of the chucking jig needs to be decreased in size, impractically resulting in a higher manufacturing cost.
As another method, there is performed a method in which solder balls are distributed into openings of a mask mounted on a substrate (so-called “distribution method”). Namely, a flux is coated on connection pads of the substrate, and then the pads and a metal mask are aligned and overlapped with each other. Then, a large number of solder balls are mounted on the metal mask, and the solder balls are slid over the mask using a flexible blade (squeegee). As a result, each of the solder balls is distributed into each opening of the mask, and is disposed on each connection pad aligned with its corresponding opening and is bonded and fixed by an adhesive flux coated on the connection pads. The solder balls remaining on the mask are collected by a collection mechanism.
The distribution method is disclosed in JP-A-10-126046, for example. Also, as a modified method thereof, JP-A-2001-320160 discloses a method in which solder balls are mounted, then a solder paste is printed and then a mask is removed. Also, JP-A-09-107045 discloses a method in which a solder paste is used in place of a flux.
In the method of mounting solder balls using a mask, in any case, it is necessary to prevent the flux coated on the connections pads on the substrate from adhering to the mask back side. When the flux adheres to the mask, the balls distributed into the mask openings are bonded to the mask. Thus, the balls are not mounted on the connection pads on the substrate. For this reason, there have been devised various countermeasures such as an increase in opening diameter of the mask, insertion of a spacer between the substrate and the mask, and use of an uneven mask.
However, when the size of each solder ball is as fine as 200 μm or less for finer pitch, unfavorably, it is difficult to prevent the adhesion of the paste onto the mask with reliability even if the foregoing countermeasures are implemented.